A gear processing machine is a machine which processes workpiece gears, and specific examples thereof include a gear profile grinding machine and a hobbing machine, which fabricate gears by cutting processing, and a gear grinding machine, which grinds gears after quenching.
When small workpiece gears are to be processed in mass production by such a gear processing machine, the workpiece gear which is the first processed product is subjected to tooth profile measurement or tooth thickness measurement, and the precision thereof is then checked. If the precision is good, remaining unprocessed lots are processed. If the precision is not good, the remaining unprocessed lots are processed after processing precision is corrected. In the gear processing machine that does not have a gear measuring function, only base tangent lengths and over-pin diameters can be checked; therefore, the first product sometimes lacks precision.
If a workpiece gear to be processed is large, a defective product is not allowed to be produced. Therefore, while machining allowance is caused to remain, processing and measurement are repeated several times, final processing precision is checked, and, then, finishing processing is carried out. Long working time is required since reattaching operation of the large gear between a gear processing machine and a gear measuring machine is required in order to repeat the processing and measurement.
The tooth profile measurement and the tooth thickness measurement with respect to the workpiece gears are carried out by a gear measuring device provided with a measuring instrument having a gauge head (probe).
Conventionally, such a gear measuring device has been generally composed as a device separated from the gear processing machine. When the gear measuring device is separated from the gear processing machine, the operation of reattaching the workpiece gear from the gear processing machine to the gear measuring device is required.
On the other hand, recently, in order to omit the above described reattaching operation to improve workability, various gear processing machines each integrally provided with a gear measuring device have been proposed so that the tooth profile measurement and the tooth thickness measurement can be carried out on the machine with respect to workpiece gears after processing (for example, see Patent Literature 1).
Even when the gear measuring device is integrated with or separated from the gear processing machine, in the gear measuring device, when a gauge head (probe) of a measuring instrument is brought into contact with a workpiece gear, a position signal indicating a position at which the gauge head is in contact with the workpiece gear is output from the measuring instrument. The tooth profile and the tooth thickness can be measured by changing the position at which the gauge head is brought into contact with the workpiece gear and subjecting the position signals at the positions to arithmetic processing.
In this case, if a position signal precisely indicating a reference position is output when the gauge head is positioned at the reference position, precise position measurement can be carried out also when measurement of other positions is carried out.
However, if thermal deformation occurs in the gear measuring device including the measuring instrument due to the surrounding temperature or heat, etc. generated upon processing of the workpiece gear, even when the gauge head is positioned at the reference position with respect to the measuring instrument, an error is generated with respect to the position with respect to the workpiece gear, and the measured position upon measurement is sometimes shifted.
If such a position error of the gauge head is generated, measurement precision is lowered when the tooth profile measurement or tooth thickness measurement is carried out. Particularly, the measurement error is large in the case in which the tooth thickness is measured.
Therefore, upon measurement, correction (calibration) of the position of the gauge head has been carried out.
A conventional calibration method will be explained with reference to FIG. 8.
FIG. 8 shows a gear measuring device 1, which measures a small or medium gear. As shown in the drawing, guide rails 3 extending along an X-axis direction, a rotating table 4, and a support column 5 are disposed on a base 2 of the gear measuring device 1.
A movable body 6 can be moved along the X-axis direction along the guide rails 3. Guide rails 7 extending in a Y-axis direction (in FIG. 8, a direction perpendicular to the paper plane) is disposed on the movable body 6, and a movable body 8 can be moved along the Y-axis direction. Guide rails 9 extending along a Z-axis direction is disposed on the movable body 8, and a movable body 10 can be moved along the Z-axis direction.
A measuring instrument 30 provided with a gauge head 31 is attached to the movable body 10.
Conventionally, in order to carry out calibration, any one of a reference block 21, a test bar 22, and a master work 23 which is a mechanical reference member is installed at a reference position determined in advance.
The reference block 21 is installed in a support arm part of the support column 5, and, in this case, the position at which the reference block 21 is installed serves as a reference position.
The test bar 22 is coaxially installed with respect to an upper surface of the rotating table 4, and, in this case, the position at which the test bar 22 is installed serves as a reference position.
The master work 23 is coaxially installed with respect to the upper surface of the rotating table 4, and, in this case, the position at which the master work 23 is installed serves as a reference position.
When calibration of the position of the gauge head 31 is to be carried out, the gauge head 31 is brought into contact with the mechanical reference member (any one of the reference block 21, the test bar 22, and the master work 23) installed at the reference position, and a position signal output from the measuring instrument 30 at this point is examined. Then, if the position signal is not indicating the reference position, calibration is carried out so that the position signal output at this point indicates the reference position.